Backlight systems, which illuminate a liquid crystal layer from behind, are in widespread use as a transmissive liquid crystal display device, and the transmissive liquid crystal display device is equipped with an edge-lit (side-lit) backlight unit, an immediate beneath type backlight unit or the like on the back face side of the liquid crystal layer. As shown in FIG. 6(a), such an edge-lit backlight unit 41 is generally equipped with a light source 42, an optical waveguide plate 43 in the form of a rectangular plate arranged with its edge aligned with the light source 42, and a plurality of optical sheets 44 superposed on the front face side of the optical waveguide plate 43. LEDs (light emitting diodes), cold-cathode tubes and the like are used as the light source 42, and the LEDs are currently in widespread use from the viewpoint of the size reduction, energy saving, and the like. In addition, as the optical sheet 44, (1) a light diffusion sheet 45 which is disposed on the front face side of the optical waveguide plate 43 and primarily has a light diffusion function, (2) a prism sheet 46 which is disposed on the front face side of the light diffusion sheet 45 and has a refraction function toward a normal direction, and the like are utilized.
The light diffusion sheet 45 substantially uniformly disperses transmitted rays of light, and is used for the purpose of achieving uniform luminance by means of the light diffusibility thereof, increasing the luminance along the front direction, etc. As shown in FIG. 6(b), such a light diffusion sheet 45 includes a substrate layer 47 made of a synthetic resin, a light diffusion layer 48 overlaid on the front face of the substrate layer 47, and a sticking preventive layer 49 overlaid on the back face of the substrate layer 47. The sticking preventive layer 49 prevents an inconvenience that a moire screen pattern is generated through sticking (adhesion) of the back face of the light diffusion sheet 45 to the front face of the optical waveguide plate 43. The sticking preventive layer 49 generally includes: spherical beads 50; and a binder 51 that covers the beads 50 and contains a thermosetting resin, and prevents the sticking thereof to the optical waveguide plate 43 by way of protruding portions that are generated so as to protrude toward the back face side due to the presence of the beads 50.
Acrylic resin beads are generally used as the beads 50 (see Japanese Unexamined Patent Application, Publication No. 2011-126274). However, since the acrylic resin beads are comparatively hard, the beads 50 protruding from the binder 51 toward the back face may scratch the front face of the optical waveguide plate 43 overlaid on the back face side of the light diffusion sheet 45. Moreover, the beads 50 may detach during processing of the light diffusion sheet 45 so as to have a shape in accordance with the design of the backlight unit 41, and the detached bead(s) 50 are/is likely to rub against the light diffusion sheet 45 and the optical waveguide plate 43, whereby the back face of the light diffusion sheet 45 and/or the front face of the optical waveguide plate 43 are/is likely to be scratched. Furthermore, in a case where given recessed portions are provided on the front face of the optical waveguide plate 43, the beads 50 are likely to fall into the recessed portions, and the contact of the recessed portions with the beads 50 is likely to generate scratches on the front face of the optical waveguide plate 43. Moreover, when the scratches are thus generated on the front face of the optical waveguide plate 43, an inconvenience that these scratches are likely to cause lack in uniformity of the luminance may be raised.